avcodec/vp8: Fix null pointer dereference in ff_vp8_decode_free()

Fixes: signal_sigsegv_d5de23_967_vp80_00_comprehensive_010.ivf with memlimit 524288

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp8.c

@@ -2689,6 +2689,9 @@ av_cold int ff_vp8_decode_free(AVCodecContext *avctx)
     VP8Context *s = avctx->priv_data;
     int i;
 
+    if (!s)
+        return 0;
+
     vp8_decode_flush_impl(avctx, 1);
     for (i = 0; i < FF_ARRAY_ELEMS(s->frames); i++)
         av_frame_free(&s->frames[i].tf.f);